Waveguide-type optical control devices are suitable for integration and a reduction in power consumption, and, thus, studies have been made on the utilization of waveguide-type optical control devices in optical switches or optical modulators. Further, in recent years, the spread of dense wavelength division multiplexing (DWDM) has lead to an increasing demand for variable optical attenuators as means for making optical powers of respective wavelengths uniform at the time of wavelength multiplexing, or as optical parts of optical ADMs (add drop multiplexers) which select a desired wavelength and inserts/removes the wavelength in a transmission line. Among others, variable optical attenuators having a directional coupler-type Mach-Zehnder (MZ) construction comprising two directional couplers provided on an LiNbO3 (lithium niobate; LN) substrate, which is advantageous from the viewpoints of a reduction in size, a reduction in voltage, and a reduction in power consumption, and a phase shifter provided between the two directional couplers are being put to practical use.
FIG. 1 shows the construction of a conventional waveguide-type optical control device having a directional coupler-type Mach-Zehnder construction. In FIG. 1, a variable optical attenuator is exemplified as the waveguide-type optical control device.
The variable optical attenuator having a directional coupler-type Mach-Zehnder construction comprises: optical waveguides 1a, 1b which are provided parallel to each other on an LN substrate (not shown); a first directional coupler 2 provided within the optical waveguides 1a, 1b; a phase shifter 3 provided adjacent to the first directional coupler 2; and a second directional coupler 4 provided adjacent to the phase shifter 3. The phase shifter 3 comprises a first electrode 3a, a second electrode 3b, and a third electrode 3c. The third electrode 3c is used as a common electrode. A negative (−) voltage is applied to this electrode from a direct current power supply 3d, and a positive (+) voltage is applied to the first electrode 3a and the second electrode 3b from the direct current power supply 3d to cause an electric field.
Next, the operation of the waveguide-type optical control device (variable optical attenuator) shown in FIG. 1 will be explained. A signal light introduced from the end of the optical waveguide 1a is branched in the first directional coupler 2 into signal light parts which are to be traveled respectively through optical waveguides 1a and 1b (branching ratio=50:50), and the branched signal lights are then input into the phase shifter 3. The phase shifter 3 operates according to the magnitude of an applied voltage 31 from the direct current power supply 3d. When the voltage 31 is not applied from the direct current power supply 3d, the branched signal lights introduced into the optical waveguides 1a and 1b are input in an identical phase into the second directional coupler 4 and the whole light is output from the output terminal of the optical waveguide 1b while no light is output from the optical waveguide 1a. 
Next, when the applied voltage 31 is increased from 0 (zero) volt, the refractive index of the optical waveguides 1a and 1b are changed and, consequently, the propagation speed of signal lights, which travel respectively through the optical waveguides 1a and 1b, is changed. Since the voltage applied to the optical waveguide 1a is opposite in direction to the voltage applied to the optical waveguide 1b, a difference occurs in propagation speed between signal light, which travels through the optical waveguide 1a, and signal light which travels through the optical waveguide 1b in the phase shifter 3. As a result, the signal light in the optical waveguide 1a and the signal light in the optical waveguide 1b are input in a mutually different phase into the second directional coupler 4. For this reason, the branching ratio (coupling rate) of the second directional coupler 4 is deviated from the original rate 50%, and, as a result, a part of signal light, which, up to this stage, has been entirely output from the optical waveguide 1b in the second directional coupler 4, is also output from the optical waveguide 1a. When the applied voltage 31 is increased to about 30 to 50 V, the signal light is substantially entirely output from the optical waveguide 1a. That is, setting the applied voltage 31 to a suitable value permits the coupling length L in the phase shifter 3 to be equivalently changed and, consequently, permits optical output corresponding to the change to be obtained.
When the voltage 31 was not applied, or when a voltage of about 30 to 50 V was applied, in order to output the whole signal light from any one of the optical waveguide 1a and the optical waveguide 1b in the second directional coupler 4, the branching ratio (coupling rate) of the first directional coupler 2 to the second directional coupler 4 should be accurately brought to 50:50 (50%). To this end, the length of a portion where the optical waveguides 1a and 1b approach each other (coupling length L=π/2γ wherein γ represents Pockels constant) should be accurately brought to the half of the complete coupling length Lc (=π/2κ wherein κ represents coupling coefficient). The deviation of the branching ratio (coupling rate) of the first directional coupler 2 to the second directional coupler 4 from 50:50 (50%) results in increased leakage of the light signal from one waveguide to the other waveguide at the output terminal of the second directional coupler 4 and thus deteriorates the ratio of the minimum attenuation level to the maximum attenuation level (extinction ratio).
FIG. 2 shows the relationship between the gap and the coupling length in a directional coupler.
The length of a portion, where the optical waveguides 1a and 1b approaches and are coupled to each other (coupling length L), and a gap G are important to the directional coupler. In order to bring the branching ratio (coupling rate) to 50:50 (50%), it is necessary to eliminate a variation in the gap G and to bring the coupling length L to [complete coupling length Lc÷2] These two are important parameters for a production process of the directional coupler.
FIG. 3 shows that characteristics vary according to the production parameters. When there is no variation in gap G shown in FIG. 2 and, at the same time, when the coupling length L is equal to the half of the complete coupling length Lc, ideal characteristics 130 are obtained, that is, the crosstalk is minimized and, consequently, the extinction ratio is increased. On the other hand, when there is a variation in gap G or when the coupling length L is not equal to the half of the complete coupling length Lc, deteriorated characteristics 131 are obtained. It is known that a change in coupling rate only by several percents from 50% causes this state.
In order to solve this problem, Japanese Patent Publication No. 72964/1994 proposes a construction such that, separately from electrodes for the phase shifter, electrodes for directional couplers are provided in the directional couplers in the optical waveguides to control the refractive index in the optical waveguides, thereby equivalently regulating the coupling length L. This construction will be explained in conjunction with FIG. 4.
FIG. 4 shows another conventional waveguide-type optical control device. Also in FIG. 4, a variable optical attenuator is used as the waveguide-type optical control device.
A first directional coupler 2, a phase shifter 3, and a second directional coupler 4 are disposed in series between the input terminal and the output terminal of the optical waveguides 1a and 1b. For applying a bias voltage, electrodes 20a, 20b are provided in the first directional coupler 2, electrodes 30a, 30b are provided in the phase shifter 3, and electrodes 40a, 40b are provided in the second directional coupler 4. The refractive index in the first and second directional couplers 2, 4 are controlled by properly setting the voltage applied to the electrodes 20a, 20b and the electrodes 40a, 40b. As a result, the coupling length L is equivalently regulated, and a deterioration in extinction ratio is improved.
Further, Japanese Patent Laid-Open No. 142569/1998 proposes a directional coupler having a construction which can reduce the level of leakage between optical waveguides and can improve dynamic range. Specifically, this publication proposes a construction which brings the coupling length L in the directional coupler to a double length of the complete coupling length Lc or a length obtained by multiplying the complete coupling length Lc by an even number.
The conventional waveguide-type optical control devices, however, have the following problems. In the construction proposed in Japanese Patent Publication No. 72964/1994 wherein dedicated electrodes are independently provided for the phase shifter and the two directional couplers, the device size of the variable optical attenuator is disadvantageously increased. Further, since the electrodes are disposed in three blocks, the number of sites where voltage control should be performed is increased. This disadvantageously complicates the construction of the control circuit.
On the other hand, according to the construction proposed in Japanese Patent Laid-Open No. 142569/1998, the coupling length L is at least twice of the complete coupling length Lc. That is, the total length of the device is long, and, thus, it is impossible to reduce the size of the device.